Normalize url

This patch implements the net (after conversation) functionality of the following two bullets on the Walker Project Plan

* Ensure we normalize URLs (NOTE: some of these may be a non-issue, just want to check them):
* Configurable parameter exclusion (case insensitive) with defaults: jsessionid, phpsessid, aspsessionid

You know, I feel like I can remember seeing JSESSIONID (rather than jsessionid)  in URLS, so I think your suggestion might be a good one. I would be inclined to think caseless purging would be better than the way it is currently, and I don't think the implementation is that much more code. Consider

```
for _, sid := range Config.PurgeSidList {
    _, found := params[sid]
    if found {
        delete(params, sid)
        needRebuild = true
    }
}
```
Changing to

```
removes := map[string]bool
for _, p := range Config.PurgeSidList {
    removes[strings.ToLower(p)] = true
}
params := u.Query()
for k := range params {
    if removes[strings.ToLower(k)] {
        delete(params, k)
    }
}
```